The present invention relates to a positioning system which includes a medical table with a table top and rotation means that can be attached to the table top and which are arranged to accommodate and rotate a part of a patient, the rotation means including supporting means for supporting the part of the patient and mounting means for mounting the rotation means on the table top.
A positioning system of this kind is known from EP-A-0 776 637 and is used in the field of stereotactic operations, notably in the field of stereotactic radiosurgery. Radiosurgery involves irradiation of cranial tumors by means of strictly collimated gamma radiation. For reasons of dosimetry it is necessary to position the tumor in the three-dimensional space in such a manner that it can be reached by the gamma radiation from different irradiation angles. The known device utilizes a stereotactic frame that is fixed to a medical table on which a patient is arranged so that the head of the patient projects from the end of the medical table. The stereotactic frame serves to fix the head of the patient and to provide a reproducible rotation of the head of the patient relative to an axis of rotation that is defined by the stereotactic frame. The known device includes a base for supporting the stereotactic frame, said base being mounted on a post on the floor. The rotary motions of the stereotactic frame as a whole are determined by the corresponding motions of the post that acts as a rotation means. Furthermore, the head of the patient can be rotated relative to the axes of rotation that are defined by the stereotactic frame. Because of the construction of the known positioning system it is necessary to use additional supporting means, that is, the combination of a base and the post, in order to support the stereotactic frame as a whole; these additional supporting means bear on the floor.
Meanwhile a need has arisen for the use of such a positioning system for the execution of MR studies of the head of the patient during a neurological intervention. The above-mentioned aspects of the known positioning system make it impossible to use this system for interventional MRI, considering the limited space available in the bore of the MR apparatus and the requirements imposed on the material, notably in respect of MR compatibility. Furthermore, the following supplementary requirements may be imposed as regards the use of the positioning system in the field of neurosurgery: first of all, the physician should have easy access to the region of interest; secondly, if rotation of the stereotactic frame as a whole is necessary, forces exerted so as to realize this rotation should be small and become manifest in the reproducibility of the rotation and the mechanical stability in the rotated position.
It is an object of the invention to provide a positioning system that can be used in the field of interventional MR where neurosurgery is a preferred intervention. To this end, the device in accordance with the invention is characterized in that the mounting means include at least one arc-shaped first guide for guiding the supporting means along the guide, the mounting means extending mainly above a lower surface of the table top.
The positioning system in accordance with the invention does not interfere with the positioning of the patient in the bore of the MR apparatus, because the mounting means of the rotation means are situated mainly above the medical table. Because the rotation means bear on the medical table, the construction of the rotation means is compact and offers the physician excellent accessibility to the cranial regions of the patient. The rotation means in accordance with the invention include the arc-shaped guides for ensuring the rotation of the head of the patient in the cranio-caudal direction.
A first embodiment of the device in accordance with the invention is characterized in that the supporting means include a second guide for co-operation with the first guide, which second guide is practically complementary to the first guide. The use of the complementary first and second guides enhances the mechanical stability of the construction as a whole, considering the enlarged bearing surface for the supporting means.
During the motion of the head of the patient in the cranio-caudal direction, a restriction may occur in respect of the space in the plane above the patient, that is in respect of the admitted volume of the MR apparatus. In order to limit the cylinder enveloping the volume of a patient, a second embodiment in accordance with the invention is characterized in that the supporting means are bent and have a concave surface that faces away from the mounting means in the assembled condition. The shape of the arc of the supporting means can be chosen in such a manner that the supporting point for the patient approaches the table top of the medical table as closely as possible, the volume of the enveloping cylinder thus being minimized. The concave shape of the supporting means provides stable positioning of the head of the patient in the transverse direction in cases where the head rests directly on the supporting means.
Investigations have shown that practically no mechanical stress is exerted on the cervical vertebrae of the patient during rotation of the head in the cranio-caudal direction if the axis of rotation for the rotation in the cranio-caudal direction coincides with the axis of rotation during nodding. Consequently, the patient as a whole need not be positioned again after completion of the rotation of the head. Therefore, for the reproducibility of the head position this rotation is advantageously performed relative to this axis. To this end, a further embodiment is characterized in that the first and the second guide are shaped as an arc of a circle. It has been found that the distance between the head base and the axis of rotation during nodding is in the vicinity of 14 cm for most humans. Therefore, a further embodiment is characterized in that the first guide and the second guide have a radius of between 10 cm and 18 cm. It is an additional advantage that the mechanical balance of the construction as a whole is then also enhanced. It has been demonstrated that the axis of rotation extends through the center of gravity of the head when the axis of rotation of the head coincides with the axis of rotation upon nodding. It is thus ensured that the head also remains balanced in the rotated position. This advantage becomes manifest in the small forces that are necessary to rotate the head, and hence also in the reproducibility of the rotation.
In order to ensure the mechanical stability of the rotation means on the medical table, it may be beneficial for the mounting means to comprise two parts. Therefore, a further embodiment of the device in accordance with the invention is characterized in that the mounting means comprise two substantially parallel first guides. When the rotation means are used in the field of neurosurgery, a stereotactic frame should be fixed on the rotation means. Therefore, a further embodiment of the device in accordance with the invention is characterized in that the supporting means include connection means for fixing a stereotactic frame on the supporting means. The supporting means include, for example a groove in the form of a dovetail, but any other mechanical solution, of course, is also feasible.
As soon as the position of the head or the stereotactic frame has been changed by means of the supporting means, it is desirable to fix the latter means in their ultimate position. Therefore, a further embodiment of the device in accordance with the invention is characterized in that there are provided first fixation means for fixing a rotary position of the supporting means in the mounting means.
Because of safety aspects it must be possible to fix the rotation means on the medical table. To this end, a next embodiment of the device in accordance with the invention is characterized in that there are provided second fixation means for fixing a position of the rotation means on the table top of the medical table.
When the supporting means are curved and approach the table top of the medical table as closely as possible, it may be that a spatial restriction arises in respect of the maximum angle of rotation that can be achieved for the supporting means. Furthermore, at a given angle of rotation the shoulders of the patient collide with the edges of the supporting means. In order to mitigate the spatial restrictions imposed by the supporting means, a further embodiment of the device in accordance with the invention is characterized in that the free edges of the supporting means that extend transversely of the first guide are provided with cut-outs, thus increasing the range of rotation of the supporting means.